What is deleted when you update software?

I noticed that iOS 4.1 software is available and I really want to download it but I am afraid. I am afraid that everything will get deleted like what happened last time. I have found a program that copies my music, videos and photos so I can use that as my media backup. But how about the notes and contacts? I have many information there and I don't want them to get deleted especially the notes. Will updating the software delete my contacts and notes? I know that it will delete media but how about the contacts and notes? Those are very important for me.